4.4 - The Sontaran Stratagem

From IMDB:
The Doctor gets a call from onetime traveling companion Martha Jones with a request that he return to Earth immediately. She and the military have taken over the ATMOS plant and she needs his help after 52 people around the world all died simultaneously. All had the ATMOS anti-pollution device fitted on their cars so the military has raided the factory believing it to be controlled by aliens. The Doctor learns that the Sontarans are planning to take over the Earth. They take Martha prisoner and she is cloned. Donna meanwhile misses her family and decides to pay them a visit.

21 out of 98 (21.4%) raters say this story requires a previous story.